What does "Verify Data" really mean?

I've been burning my first DVDs using Nero Express 6.6.1.4 on a Memorex burner with Maxell 16X DVD-R's. I've tried burning about 9 DVDs - 3 successfully at 16X but then I started getting errors so I tried 12X which also failed and now I'm using 8X. At 8X I've had two failures. One failed during the burn, one failed the verify data check. A couple others are OK except that one will not mount in the burner even though it passed the data verify check.

So, what does passing the data verify check mean? Is there data needed for the disk to mount but which is not checked during the verify pass?

The correct way to verify is to open the tray and then close it, to check that the disc can be mounted. Nero does not do this in most cases. The following option can help: NBR > Options > Expert Features > Do not check "Do not eject the disc after burn is completed".